feat: Bundle UI into coderd; add ./develop.sh script (#28)

This change bundles the static assets like we have for v1 - using the [`embed`](https://pkg.go.dev/embed) go package. Fixes #22 

In addition, it sets up a development script that runs `coderd` locally and serves the front-end, with hot-reloading. The script used is `./develop.sh`:

![2022-01-14 17 30 14](https://user-images.githubusercontent.com/88213859/149603926-f673d3d3-ba12-4eda-bcdd-427252405480.gif)

> NOTE: The UI is still placeholder, of course. Need to start testing out a simple, placeholder flow for the new v2 world as a next step

Summary of changes:
- Add build steps for `go` in the `Makefile`
  - Add a step for production build, in which we use the `embed` tag
  - Add a step for development, which doesn't need the `embed` tag - so we don't need to build the front-end twice
- Add `next export` build step to output front-end artifacts in `out`
- Add a `site` package for `go`
  - Add `embed_static.go` and `embed.go`. This is mostly brought in as-is from v1, except removing some intercom/sentry CSP entries that we aren't using.
- Add a [next development server](https://nextjs.org/docs/advanced-features/custom-server)
- Add a `v2-dev` script, that runs `coderd` and the `next` dev server side-by-side
- Use the `site` package as the fallback handler.
- Add `.gitignore` entries for additional build collateral

package.json

@@ -6,7 +6,8 @@
   "scripts": {
     "build": "NODE_ENV=production next build site",
     "build:dev": "next build site",
-    "dev": "next dev site",
+    "dev": "ts-node site/dev.ts",
+    "export": "next export site",
     "format:check": "prettier --check '**/*.{css,html,js,json,jsx,md,ts,tsx,yaml,yml}'",
     "format:write": "prettier --write '**/*.{css,html,js,json,jsx,md,ts,tsx,yaml,yml}'",
     "test": "jest --selectProjects test",
@@ -17,11 +18,14 @@
     "@material-ui/icons": "4.5.1",
     "@material-ui/lab": "4.0.0-alpha.42",
     "@testing-library/react": "12.1.2",
+    "@types/express": "4.17.13",
     "@types/jest": "27.4.0",
     "@types/node": "14.18.4",
     "@types/react": "17.0.38",
     "@types/react-dom": "17.0.11",
     "@types/superagent": "4.1.14",
+    "express": "4.17.2",
+    "http-proxy-middleware": "2.0.1",
     "jest": "27.4.7",
     "next": "12.0.7",
     "prettier": "2.5.1",
@@ -29,6 +33,8 @@
     "react-dom": "17.0.2",
     "ts-jest": "27.1.2",
     "ts-loader": "9.2.6",
+    "ts-node": "10.4.0",
     "typescript": "4.5.4"
-  }
+  },
+  "dependencies": {}
 }